With the explosive growth of mobile traffic demand, the contradiction between capacity requirements and spectrum shortage becomes increasingly prominent. The fifth generation (5G) of wireless networks will have to address this problem. An increasingly popular solution is to incorporate the millimeter wave (mmWave) band, which runs from 30 GHz to 300 GHz, into these 5G networks. While the huge bandwidth in the mmWave would allow for accommodation of more mobile traffic, fundamental differences between current systems operating in the microwave band, which runs from 2.4 GHz to 5 GHz, introduce new problems such as high propagation loss, directivity, sensitivity to blockage, and dynamics due to mobility of mmWave communications. In parallel, there is also a rapidly increasing demand on higher data rate for the communication systems in existing GHz frequency regime. All these challenges require new thoughts and insights in architectures and protocols.
The power amplifier (PA) serves as the interface between the radio frequency (RF) transmitter system and the antenna and is often considered one of the most critical building blocks in a wireless network. Due to the PAs effects on the efficiency and linearity of a network, they will play a critical role in the future wireless communication networks operating at the mmWave bands as well as the GHz bands.
Conventional PAs are fully analog PAs, fully digital PAs (DPA), or DPAs with some minor analog linearization. Fully analog PAs are typically employing large power devices and suffer from a direct tradeoff between PA efficiency and linearity performance. As most fully analog PAs are designed to achieve peak efficiency at maximum power output, their power-back off (PBO) operations, which accommodate for the large peak-to-average power ratios (PAPRs) and increase linearity, will degrade the efficiency. Alternatively, fully analog PAs that reduce PBO levels in order to increase efficiency will sacrifice linearity. Moreover, fully analog PAs only allow limited controls on the power cell operations, which restricts their use and achievable performance in various advanced PA architectures, such as Doherty PAs. On the other hand, fully digital PAs are composed of an array of weighted PA cells, and they maintain an advantage over fully analog PAs in that they can precisely control individual PA cells in order to increase the linearity. However, fully digital PAs synthesizes the desired output amplitude purely by digitally controlling the weighted PA cell array, which sets a fundamental limit on the accuracy to interpolate the desired PA output amplitude and the minimal amplitude that the PA can interpolate, i.e. the quantization error that is governed by the least-significant-bit (LSB) of the digital PA cell array. Therefore, to achieve large output dynamic range and low amplitude quantization error, the fully digital PA should employ a large number of weighted PA cells and thus a large number of amplitude control bits. In reality, for high speed modulations, it is increasingly challenging to accurately generate these large number of amplitude control bits and ensure good timing synchronization among them and the synchronization with other signal paths, e.g., the phase modulation path. Any timing mismatch or control bit error will inevitably lead to linearity degradation. In addition, reported DPAs with minor analog linearization only employ analog linearization for the output phase signals, and they are still governed by the aforementioned amplitude quantization errors and output dynamic range set by the finite number of weighted PA cells in the PA.
Therefore, there exists the need for a new PA architecture that can achieve high linearity, high efficiency, and well-controlled PA power cells, whilst employing limited number of control bits, in order to enable the next generation wireless communication systems.